1. Field of the Invention
The present invention relates to the separation of isotopes by irradiation.
2. Description of the Prior Art
The application of irradiation by means of lasers to atomic separation has been described in the article of Ashkin in Physical Review Letters, 25, (19) of Nov. 9, 1970, pages 1321-1324. According to Ashkin it is proposed to use the saturated value of the radiation pressure force on neutral atoms to produce a central force field to deflect atoms in circular orbits and make a high velocity analyser. This is stated to be useful for studying the interaction of atoms with high-intensity monochromatic light and to separate, velocity analyze, or trap neutral atoms of specific isotopic species or hyperfine level. In addition, C. Bradley Moore in a publication, Accounts of Chemical Research, 6 (1973) 323-8, reviews the application of lasers to isotope separation.
The process of the present invention is similar in certain respects to the process of Ashkin, but differs therefrom in some critical aspects, resulting in a considerably higher separation factor and efficiency. According to the process of Ashkin the atoms are irradiated from one direction only, and they are accelerated in one direction only. An atomic beam emerging from an over into a vacuum is irradiated with saturating resonance radiation transverse to the initial velocity of the beam. When an atom absorbs a photon, of energy h.nu. the momentum of the atom increases by a vector of magnitude h.nu. /c and direction parallel to the motion of the photon prior to the absorption. The excited atoms emits spontaneously a photon, on the average in a random direction, and thus gains on the average zero additional momentum. The emission of the photons takes place after a certain time lag, the average of which is the lifetime of the excited atom. The direction of the emitted photons in space is random. On the average each atom gains a momentum of h.nu. /c along the direction of the laser beam per each absorbtion and emission of a photon. This process is repeated as long as the atom moves within the laser beam, and per each absorption and emission a slight deflection of the beam is attained. When the beam is directed on a mixture of two or more isotopes of a certain element, one of these can be selectively deflected by using laser light in resonance with the one isotopic species but not with the others, and only these atoms will be deflected. Tunable dye lasers make possible the exact adjustment of the wave-length required for the intended purpose. The Ashkin method has a number of drawbacks: The power requirement is rather high. For example, to deflect by one degree of arc an atomic beam having a flux of 1 ton per year, of atoms moving at a thermal speed of 3.104 cm/secl, an average laser power of 50 KW throughout the year is required. Since the atomic beam is narrow in the direction of propagation of the laser beam, it has a small optical density, and thus a significant part of the photons traverse the atomic beam without being absorbed. This results in a further loss of power and increases substantially the costs of the separation. Furthermore, the laser power per unit area must be kept low enough to avoid the phenomenon of induced emission, since induced emission along the laser beam results in a loss of the momentum of the atoms gained by the absorbtion of the photon from the laser light. The method of Ashkin is not applicable to atoms having intermediate metastable (very long lived) states. If atoms pass into such metastable state, they are unable to absorb further laser light since the laser is tuned only to the transition from the ground state to the desired excited state. In the context of the present discussion, a metastable state is one of the order of magnitude of the passage time of the atom across the laser beam. This is typically of the order of 10 .sup.-.sup.4 seconds. Uranium atoms have quite a number of metastable states and thus it is not certain that the method proposed by Ashkin will be applicable to uranium ions. Furthermore, the method proposed by Ashkin provides only random direction of the momentum due to emission of the photons.